1. Field of the Invention
The present invention relates to ceramic turbine disk assemblies and more particularly to the use of an electroformed compliant layer at the interface of a ceramic turbine blade and a turbine disk.
2. Description of the Prior Art
In a number of government-funded programs Si.sub.3 N.sub.4 and SiC hardware are being successfully tested for use in the hot gas path of gas turbine engines. Success of these nonoxide components can be attributed to improved formulations and methods for fabricating shapes with high strength and good resistance to oxidation. Design methodology for brittle materials has also advanced towards the goal of improving reliability of ceramic components in turbine engines.
However, the design and fabrication of ceramic-to-metal and ceramic-to-ceramic joints remains a problem. Such joints are prone to failure because ceramics do not yield locally, as do metals, and critical stresses can readily develop in the ceramic assembly at the point of contact.
An example is a ceramic turbine blade with a metal disc joint. The conventional fir tree root configuration used on metal blades cannot be normally used for ceramics because ceramics do not yield at contact points to spread the load over a larger surface. On the contrary, critical stresses are developed in the ceramic and failure of the ceramic results.
Two methods have been pursued for spreading the contact zones between ceramic and metal surfaces over larger areas. One is to forge the metal disc around the ceramic blade roots. Disadvantages include complexity of the fabrication process, limited alloy selection, and possible damage to the blade during processing. The more popular method for enlarging the contact loading area is to insert a compliant layer of ductile material between the ceramic root and the surface of the slot in the metal disc. Compliant materials are selected to yield sufficiently at service temperature to increase contact area but to not yield so much that the ceramic root touches the metal disc. Among their disadvantages, however, is that they do not assure intimate and continuous surface contact especially under loading such that their ability to evenly distribute local stress about a ceramic blade is impaired. Compliant layers include metal foils of such alloys as L605 and Haynes 188, and certain glasses.